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Closed-die forging and slab hot rolling with focus on material yield: some industrial problems analysed by FEM
KTH, School of Industrial Engineering and Management (ITM), Production Engineering.
2008 (English)Doctoral thesis, comprehensive summary (Other scientific)
Abstract [en]

The thesis is focussed on improving the material yield in closed-die forging and rolling. The former is restricted to the manufacturing of heavy crown wheels and front axle beams and the latter to the hot rolling of steel slabs. To enhance the yield the commercial FE-codes Form2D and Dyna3D are used. Results from forging simulations are strengthened by full scale experiments. The research is carried out in near contact with the Swedish steel and engineering industry. In closed die forging, two bulk forming problems are treated: How to improve the tool design and how to change the pre-form geometry for decreasing the amount of material exiting the flash gap? In slab rolling, two problems related to material defects are considered: How to eliminate existing surface cracks and how to prevent the formation of voids around macro-inclusions embedded in the steel matrix? Internal voids might be the reason for scrapping the whole workpiece.

Considering the forging of crown wheels, a new concept is proposed. For making the central hole of the product, the traditional method was forging a thin circular plate which was then sheared off and scrapped. Using the new technique this operation is replaced by forging a conical tap in the centre, which is then discarded. Doing so, the inner scrap material decreased with about 15%. The idea has been used in production for seven years. – Regarding the forging of front axle beams, a quasi-3D method is used comprising full scale measurements of the axial material flow. Here the theoretical material yield increased 2-7%.

Regarding rolling, the initial surface cracks in the simulations are V-shaped with a crack angle of 6o and of depth 5-20 mm. The inclusions are cylindrical and either three times harder or three times softer than the matrix. The behaviour of the cracks and the inclusions are studied as influenced by process parameters. Current industrial input data are used as a reference. – It is concluded that longitudinal cracks cannot be totally eliminated. During rolling their V-shape gradually changes to Y-shape and a remaining oxide flake separates the crack bottom surfaces. For minimizing the detrimental influence of the entrapped oxide, an early closure of the crack bottom is important. Throughout the remaining rolling schedule the entrapped oxide is then torn to pieces with large areas of virgin metal in between ensuring a strong bond. The following recommendations are given for the longitudinal cracks: Light drafts/pass at the beginning of the rolling schedule followed by heavy ones. – Contradictory to longitudinal cracks it is concluded that transversal cracks are possible to eliminate. When eliminated, the initial bottom of the crack coincides with the slab surface. No folds are formed. For the transversal cracks light drafts/pass are proposed together with reversal rolling, the latter for avoiding crack folding. – Voids are easiest formed around hard macro-inclusions in the centre of the slab. Large rolls and heavy drafts are recommended to avoid this.

Place, publisher, year, edition, pages
Stockholm: KTH , 2008. , 41 p.
Series
Trita-IIP, ISSN 1650-1888 ; 08-01
Keyword [en]
Material yield, Closed-die forging, Hot rolling, Cracks, Macro-inclusions, Steel, FEM
National Category
Production Engineering, Human Work Science and Ergonomics
Identifiers
URN: urn:nbn:se:kth:diva-4690ISBN: 978-91-7178-871-9 (print)OAI: oai:DiVA.org:kth-4690DiVA: diva2:13454
Public defence
2008-04-25, Sal M311, KTH, Brinellvägen 68, Stockholm, 10:00
Opponent
Supervisors
Note
QC 20100720Available from: 2008-04-03 Created: 2008-04-03 Last updated: 2010-07-20Bibliographically approved
List of papers
1. A new closed-die forging concept for the manufacturing of crown wheels
Open this publication in new window or tab >>A new closed-die forging concept for the manufacturing of crown wheels
1999 (English)In: Advanced Technology of Plasticity 1999: Proceedings of the 6th International Conference on Technology of Plasticity Nuremberg, September 19 - 24, 1999, Vol. III / [ed] M. Geiger, 1999, 1663-1668 p.Conference paper, Published paper (Refereed)
Keyword
hot forging; crown wheel; material yield
National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-8193 (URN)978-3-540-66066-8 (ISBN)
Conference
6th International Conference on Technology of Plasticity Nuremberg, September 19 - 24, 1999
Note
QC 20100720Available from: 2008-04-03 Created: 2008-04-03 Last updated: 2010-07-20Bibliographically approved
2. A quasi-3D method used for increasing the material yield in closed-die forging of a front axle beam
Open this publication in new window or tab >>A quasi-3D method used for increasing the material yield in closed-die forging of a front axle beam
2005 (English)In: Journal of Materials Processing Technology, ISSN 0924-0136, Vol. 160, no 2, 119-122 p.Article in journal (Refereed) Published
Abstract [en]

The aim of the work is to increase the material yield in closed-die forging of a front axle beam meant for heavy trucks. The amount of flash obtained in production constituted 35% of the total workpiece weight, which was equal to 115.4 kg. The strategy for improving the material yield was to modify the initial forging workpiece geometry, keeping the pre- and finishing die geometries constant. Recommending new shapes for certain cross-sections of the reducer-rolled billet did this. The goal was reached by using a quasi-3D analysis. Three critical cross-sections that showed close to plane strain conditions during forging were chosen for the analysis. The FE-code Form2D was used. The losses of material in the sections caused by axial material flow were measured from full-scale experiments and added to the optimised cross-sectional areas established from the FE-analysis. Because of advice front industry the initial cross-sections used in the 2D-forging simulations were chosen to be circular. Also the final results, after compensation for the axial material flow are presented as circular cross-sections. Utilising the results from the quasi-3D approach the theoretical material yield was increased by 2.58-7.59% for the cross-sections. Results from this work have facilitated the development of adequately shaped grooves in the reducer rolling mill so that the flash volume obtained in production has been reduced. The investigation was carried out with data from IMATRA KILSTA AB.

Keyword
quasi-3D; material yield; closed-die forging; front axle beam; FE-analysis
National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-8194 (URN)10.1016/j.jmatprotec.2004.05.007 (DOI)000227689700003 ()2-s2.0-13844297035 (Scopus ID)
Note
QC 20100720Available from: 2008-04-03 Created: 2008-04-03 Last updated: 2010-07-20Bibliographically approved
3. Behaviour of longitudinal surface cracks in the hot rolling of steel slabs
Open this publication in new window or tab >>Behaviour of longitudinal surface cracks in the hot rolling of steel slabs
1999 (English)In: Journal of Materials Processing Technology, ISSN 0924-0136, Vol. 94, no 2-3, 141-150 p.Article in journal (Refereed) Published
Abstract [en]

The behaviour of a longitudinal V-shaped crack, on the surface of a continuously cast steel slab, is studied during hot rolling. The analysis is carried out by means of the commercial FE-code LS-DYNA3D. Process parameters obtained from industry are used as a reference. The slab of initial width 1000 mm and thickness 220 mm is rolled down to 30 mm. It is assumed that the material can be treated as rigid-perfectly plastic and that the cracks do not propagate. The latter assumption is in agreement with industrial observations for a steel grade similar to that analysed here. The aim of the study is to investigate the possibility of controlling the plastic deformation so that the cracks disappear or so that their deteriorating effects are minimised. The analysis is focused upon the influence of friction, roll radius and rolling schedule on the change in the shape of a crack of initial depth 20 mm and a crack angle of 6 degrees. The reliability of the simulations is checked by pilot-plant experiments using aluminium as the model material for steel.

The results indicate that it is not possible to prevent the bottom side surfaces of the crack from coming into contact, especially not for small reductions/pass and small roll radii. The influence of friction was found to be marginal. Contact between the crack surfaces is found already at the beginning of the rolling, as the V-shaped crack is being changed to Y-shape. Considering the upper part of the crack, this remained open during the majority of the schedules studied. However for heavy reduction/pass and a large roll radius, this part of the crack is closed also, but not before the final passes. If the bottom side surfaces of the crack are in complete contact, they are prevented from further oxidisation. It is assumed that for such conditions the deteriorating influence of the bottom part of the defect decreases during subsequent rolling. During the elongation of the workpiece, the oxide flake of the Y-crack bottom is broken into splinters with oxide free material in between, making the creation of a high performance weld possible. Provided that this supposition is correct, the best results should be obtained for Light reductions/pass at the beginning of the rolling, resulting in an early closure of the crack bottom, followed by heavy reductions/pass enabling also the closure and oxide splintering of the upper part of the folded crack.

Keyword
longitudinal surface cracks; hot rolling; steel slabs
National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-8195 (URN)10.1016/S0924-0136(99)00090-4 (DOI)000083300600010 ()
Note
QC 20100720Available from: 2008-04-03 Created: 2008-04-03 Last updated: 2010-07-20Bibliographically approved
4. Transversal cracks and their behaviour in the hot rolling of steel slabs
Open this publication in new window or tab >>Transversal cracks and their behaviour in the hot rolling of steel slabs
2000 (English)In: Journal of Materials Processing Technology, ISSN 0924-0136, Vol. 101, no 1-3, 312-321 p.Article in journal (Refereed) Published
Abstract [en]

The behaviour of transversal cracks in reverse rolling of steel slabs is analysed. For this purpose a commercial FE-program, LS-Dyna3D has been utilised. Process parameters from an industrial rolling mill have been used as a reference. One basic assumption is that the cracks do nor propagate for the steel considered. This is in agreement with observations in industry. Furthermore the material is treated as elastic-plastic. The aim of the study is to investigate the possibility of controlling the plastic deformation so that the cracks disappear at the same time as their boundary surfaces are prevented from getting into contact with each other. If they should get into contract, a defect in the form of an oxide flake should be created. The optimum situations are assumed to be found when the depth of the crack decreases more rapidly than the height of the slab at the same time as the crack width increases. For such rolling conditions it should be possible to find the crack bottom on the workpiece surface without any trace let by the original crack.

During rolling two phenomena are present, counteracting each other. At the entrance to the roll gap, the crack widens, which is regarded as beneficial. The explanation to the phenomenon is that when the front edge of the crack gets into contact with the roil it is dragged towards the gap at the same time as its velocity is rapidly changed from horizontal to become parallel to the tangential velocity of the roll. If this phenomenon did not occur, the risk for the creation of a permanent oxide Rake should be big. During the passage of the gap the widened crack angle is decreased. The decrease of the angle is explained by the high hydrostatic pressure that is associated with the roll/workpiece contact surface. At the exit the angle is slightly increased once again which might be explained by the fact that the back edge of the crack is still in contact with the roll and its movement is retarded because of friction. Thus the final result depends on which of the two phenomena, widening or contraction that has been dominating.

The analysis shows that light reductions/pass, small roll radius and high friction are preferred. A deep crack of V-shape is easily transformed to Y-shape. This means that a remaining defect should be formed at the bottom of the crack. Reverse rolling is found to be beneficial because this way of rolling implies that the crack does nor tilt so easily which should result in the formation of an oxide flake.

Keyword
transversal cracks; steel slabs; industrial rolling mill
National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-8196 (URN)10.1016/S0924-0136(99)00481-1 (DOI)000086418100044 ()
Note
QC 20100720Available from: 2008-04-03 Created: 2008-04-03 Last updated: 2010-07-20Bibliographically approved
5. Void initiation close to a macro-inclusion during single pass reductions in the hot rolling of steel slabs: a numerical study
Open this publication in new window or tab >>Void initiation close to a macro-inclusion during single pass reductions in the hot rolling of steel slabs: a numerical study
2005 (English)In: Journal of Materials Processing Technology, ISSN 0924-0136, Vol. 170, no 1-2, 142-150 p.Article in journal (Refereed) Published
Abstract [en]

The work is focussed on the start of void formation close to a hypothetical single macro-inclusion in the hot rolling of steel slabs. The inclusion is chosen to be either three times harder or three times softer than the surrounding matrix. A commercial FE-code LS-Dyna3D has been utilized for the numerical analysis. Process data from the Swedish company "SSAB Tunnpl (a) over circlet AB" are used. The initial inclusion geometry is assumed to be cylindrical with an extension through the entire width of the slab. Different distances from the slab surface, on the vertical symmetry plane in rolling direction, are chosen as locations for the enclosure. Influence of different single pass reductions and roll radii are analysed. The only boundary force used on the matrix/inclusion interface is referred to friction. By treating a very big inclusion and neglecting bonding forces in radial direction, a worst-case scenario means to be studied. Two indices, one describing the deformation of the inclusion and one the surrounding matrix are used to estimate the likelihood for start of void formation. If the matrix index is bigger than that of the inclusion, voids are assumed to be initiated. This hypothesis gives trusty results. Near the hard particle, voids are likely to be formed. It became clear that the risk is smaller when large rolls are used. Furthermore, the threat for start of void formation turned out to be slighter for high reductions and in the vicinity of the slab surface. Soft particles were just elongated and no risk for void formation could be predicted.

Keyword
void initiation; macro-inclusion; inclusion location; hot rolling; steel slab; reduction; roll radius; FE-analysis
National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-8197 (URN)10.1016/j.jmatprotec.2005.04.117 (DOI)000233438700020 ()2-s2.0-27344434119 (Scopus ID)
Note
QC 20100720Available from: 2008-04-03 Created: 2008-04-03 Last updated: 2010-07-20Bibliographically approved

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Citation style
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